Power-assist transmissions, which are becoming increasingly popular, are structurally similar to a conventional manual transmission, except that the user-operated clutch pedal and gear lever are replaced by corresponding electric or hydraulic servocontrols. In a power-assist transmission, the user simply sends an up- or downshift order to a transmission central control unit, which independently shifts gear by acting on both the engine and the clutch and gearbox servocontrols.
A gear-shift order may be generated manually, i.e. by the driver, or automatically, i.e. independently of the driver. When a gear-shift order is generated, the transmission central control unit drives the clutch servocontrol to release the clutch and separate a primary shaft of the gearbox mechanically from a drive shaft, and simultaneously acts on the engine central control unit to temporarily reduce the drive torque generated by the engine.
On determining release of the clutch, the transmission central control unit drives the gearbox servocontrols to disengage the currently engaged gear; on determining disengagement of the gear, the transmission central control unit drives the gearbox servocontrols to move the primary shaft with respect to a secondary shaft and so prepare to engage the new gear; and, on determining the desired position of the primary shaft with respect to the secondary shaft, the transmission central control unit drives the gearbox servocontrols to engage the new gear.
Finally, on determining engagement of the new gear, the transmission central control unit drives the clutch servocontrol to engage the clutch and so connect or couple the primary shaft of the gearbox and the drive shaft angularly integral with each other, and simultaneously acts on the engine central control unit to restore the drive torque generated by the engine.
It is important to note that the transmission central control unit and the engine central control unit currently communicate over an on-vehicle, typically CAN protocol, BUS line. In other words, using the on-vehicle BUS line, the transmission central control unit sends a first message to the engine central control unit requesting it to cut off the drive torque generated by the engine, and subsequently sends a second message to the engine central control unit requesting it to restore the drive torque generated by the engine. To prevent the wrong commands being performed, a normal copy followed immediately by a complemented copy of each message is transmitted; and the engine central control unit compares the two successive in-coming messages, and only performs the command if the two messages are consistent.
The BUS fails to provide for precise synchronization, on account of the randomly variable time lapse, normally ranging between 20 ms and 40 ms, between the instant a message is sent by the transmission central control unit and the instant the relative command is performed by the engine central control unit. As a result, after sending a message to the engine central control unit, the transmission central control unit must determine performance of the command contained in the message to determine the exact timing with which the command is performed.
In normal driving conditions, a power-assist transmission is expected to shift gear rapidly with no sharp longitudinal acceleration/deceleration of the vehicle, which, besides annoying the occupants of the vehicle, subjects the transmission to unnecessary mechanical stress. And in racing mode, a power-assist transmission is expected to shift gear as fast as possible without subjecting the transmission to excessive mechanical stress.
Currently marketed power-assist transmissions operating as described above normally take from 250 to 600 ms to shift gears. Obviously, the actual time taken depends on dynamic performance of the transmission components and the degree of comfort demanded. A 250 ms gear shift is in itself fast, but is still relatively slow in racing mode and particularly in competition racing mode.
EP0512708A2, which is incorporated by reference, discloses a control method/system for minimizing the time required to complete a compound power downshift of an automated transmission system comprising a compound range type transmission provided with a power synchronizer effective only when the auxiliary section is engaged is provided.
EP0355070A1, which is incorporated by reference, discloses a torque modulation control process for a thermal engine associated with an automatic transmission, wherein the electronic control cabinet of the automatic transmission acts on a control member of the engine by means of a one-way and single-wire connection by means of a torque modulation signal which is an all-or-nothing signal of modulated duration, so as to act on the engine torque during gear changes.